Fabrication and Characterization of Bulk Heterojunction Solar Cells Based on Liquid-Crystal Semiconductive Polymer ()
1. Introduction
Organic solar cells have their unique advantages of useful applications, low cost, light weight and easy processing [1,2]. The organic solar cells based on a low band gap conjugated polymer and fullerene have been studied in recent years [3,4]. Advantages of liquid crystal semiconductive polymer have spontaneous self-assembly, relatively high charge mobility, easy deposition by spin coating, roll to roll or ink-jet printing. For instant, liquid crystal semi-conductor polymer of poly(9,9-dioctylfluoreneco-bithiophene) (F8T2) as a block copolymer with alternating dioctylfluorene and bithiophene segments have been applied as hole transporting layer, organic fieldeffect transistors and photovoltaic system [5]. The F8T2 copolymer with bithiophene segments affords good holetransporting properties [6]. The liquid crystal semiconductor polymer of F8T2 depending on thermal treatment has been applied electron devices with charge transfer based on molecular interaction of molecular self-coagulation [7].
Bulk heterojunction organic solar cell of F8T2 and fullerene as p-type and n-type semiconductors has been studied for improving photovoltaic and optical properties. A significant improvement of the photovoltaic performance has been reported by using bulk heterojunction thin film in the wide range of spectra. Weight ratio of composition in active layer has been varied to investigate relationship between morphological behavior and the photovoltaic properties [8,9]. Control of morphology of the bulk heterojunction film is important for tuning in optimizing exciton diffusion, charge separation, and electron (hole) transfer to cathode (anode). The photovoltaic properties have been an influence on molecular ordering in crystal phase controlled by heat treatment at glass temperature and liquid crystal transition. Crystallographic structure and morphological behavior of bithiophene-fluorene copolymer and fullerene varied with mole ratio of segment has been studied for improving the photovoltaic performance [9,10].
The purpose in this study is to fabricate and characterrize bulk heterojunction organic solar cell based on liquid crystal semiconductive polymer of F8T2 and fullerenes (C60) as electron donor and acceptor. Relationship between the photovoltaic properties and morphological behavior will be focused on tuning for optimization of photovoltaic performance. The thermal behavior on morphological behavior and molecular ordering of the liquid crystal polymer of F8T2 mixed with C60 in the active layer will be investigated by polarized optical mi-croscopy, atomic force microscopy (AFM) and Raman scattering spectra. Mechanism of the photovoltaic and optical properties will be discussed on the basis of experimental results.
2. Experimental
Poly(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene (F8T2), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and C60 were used as received from Aldrich Co. Ltd. and Material technologies Research. The molecular structure of C60 and F8T2 were shown in Figures 1(a) and (b). The number and weight average molecular weight of F8T2 was Mn > 20,000 and Mw = 41,126, as reported by Aldrich Co. Ltd. Mole ratio of (9,9-dioctylfluorenyl-2,7-diyl) and (bithiophene) element parts in F8T2 was 1 to 1, which indicates alternative copolymer. A repeat of cleaning ITO (A11DU80, AGC Fabritech Co. Ltd. 2 × 2 cm, 10 Ω/sq.) was taken by organic solvents such as acetone, methanol and distilled water. The ITO substrate was dried by N2 gas. The ITO substrate was irradiated with UV lamp for 30 minutes. PEDOT:PSS was spin-coated on the cleaned ITO substrate in glove box under N2 atmosphere. Heat treatment was carried out at 100˚C for 20 min in N2 atmosphere.
Bulk heterojunction films of F8T2 (10 mg) and C60 (10 mg) solved in o-dichlobenzene (1.0 mL) was prepared on the PEDOT:PSS film based on the ITO substrate by spin coating (MIKASA SPINCOATOR 1H-D7). Heat treatment was carried out at several temperatures for 25 min. The substrate was cooled down in N2 atmosphere. Aluminum (Al) metal was evaporated at thickness of about 100 nm in an area of 0.16 cm2 on a top of the organic layer. Figure 1(c) shows schematic diagram of the bulk heterojunction solar cells of F8T2/C60.
Figure 1. Molecular structures of (a) C60 and (b) F8T2. (c) Schematic diagram of bulk heterojunction solar cells.
Light and dark current density voltage (J-V) characteristics (Hokuto Denko Corp., HSV-110) of the solar cells mentioned above were measured under AM 1.5 (100 mW×cm–2) irradiation (Sanei Electric, XES-301S) in N2 atmosphere. Optical properties of the heterojunction film were measured by UV-vis spectroscopy (Hitachi U-4100). Surface morphology of the active layer was observed by AFM (SII Nano Technology Inc. SII SPA400). Thermal behaviors of the F8T2 films were measured by DSC (PERKIN ELMER DSC Pyris 1). Raman scattering spectra were recorded with a Laser Raman Spectrometer (NRS-5100, JASCO Co., Ltd.). Raman mode and optical image of the active thin film after annealing at 70˚C was observed using excitation laser wavelength at 532 nm.
The molecular structure of F8T2 monomer was optimized by CS Chem3D (Cambridge Soft). Molecular orbital calculations were carried out by MOPAC (Fujitsu Ltd.). The isolated molecular structures were optimized by ab-initio quantum calculation using density functional theory using B3LYP/6-31G (d) as basis function (Gaussian 03). The electronic structures with energy levels at HOMO and LUMO were calculated. Active modes in Raman scattering spectra were calculated by DFT/ B3LYP/6-31G (d) using frequency mode.
3. Results and Discussion
Thermal behaviors in the range of glass temperature and liquid crystal transition of the F8T2 polymer films were investigated as shown in Figure 2. Thermal transition for F8T2 was conformed to be at 118.5˚C, 267.5˚C and 305.5˚C, which were suitable for a glass temperature (Tg), liquid-crystal transition and isotropic transition. These values were closed to be transition points, 128.5˚C, 259.3˚C, 314.1˚C, as reported in previous literature [5]. The annealing treatment at several temperatures was carried out for improving the photovoltaic properties and molecular ordering in active layer.
The photovoltaic performance including current voltage curves in the dark and illumination of the F8T2/C60 bulk heterojunction solar cells were measured as shown in Figure 3. In the case of annealing condition at 70˚C below glass temperature, the light-induced J-V curves in the bulk heterojunction solar cell displayed a slight increase of photo-induced current density in dependence on applied voltage. The light-induced J-V curves of the solar cells indicated the photovoltaic behavior, which had a light induced rectification in strain diode characterization with exponential function of light induced current depending on applied voltage. The photovoltaic performance was improved by annealing treatment at 70˚C and 100˚C below glass temperature. However, the performance was reduced with arising annealing temperature at 130˚C and 190˚C below liquid crystal transition.
Figure 2. Thermal behavior of F8T2 film by DSC.
Figure 3. Current voltage curves in illumination of F8T2/ C60 bulk heterojunction solar cells after annealing.
Table 1 shows measured parameters varied with the annealing condition. In the case of the photovoltaic characterization after annealing condition at 70˚C and 100˚C below the glass temperature, the measured parameters, open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and light conversion efficiency (η) were obtained to be 0.75 V, 0.55 mA×cm–2, 0.16, 6.8 × 10–2% for 70˚C, 0.37 V, 0.48 mA×cm–2, 0.27 and 4.8 × 10–2% for 100˚C, respectively. At 130˚C and 160˚C, the experimental results indicate a slight increase of photoinduced current density depending on the voltage, the measured parameters, Voc, Jsc and FF were obtained to be 2.4 × 10–2 V, 5.3 × 10–3 mA×cm–2, 0.15, for 130˚C and 0.48 V, 6.8 × 10–3 mA×cm–2, 0.16 for 160˚C. The conversion efficiency, η was estimated to be 1.9 × 10–5% and 5.4 × 10–4%, respectively. In contrast case, device parameters for F8T2/C70 bilayer were referred to be 0.76 V, 3.07 mA×cm–2, 0.43 for 100˚C, 0.67 V, 9.55 mA×cm–2 and 0.53 for 200˚C [11]. The conversion efficiency, η was reported to be 1.2% and 3.4%, respectively. The photovoltaic performance for the F8T2/C70 bilayer was origi-